Materials Science and Engineering A 467 (2007) 132–138 Effect of scandium additions on microstructure and mechanical properties of Al–Zn–Mg alloy welds Selvi Dev a,∗ , A. Archibald Stuart a , R.C. Ravi Dev Kumaar a , B.S. Murty b , K. Prasad Rao b a Saipem – ENI Group (StarStroi), Y. Sakhalin, Russia b Indian Institute of Technology, Chennai, India Received 3 September 2006; received in revised form 20 February 2007; accepted 21 February 2007 Abstract The microstructure and mechanical properties of fusion zones of medium strength Al–Zn–Mg alloy (RDE-40) welds obtained by using different fillers containing various amount of scandium was investigated. It was observed that addition of scandium led to very significant grain refinement in the fusion zone especially for scandium levels greater than the eutectic composition (0.55 wt%). The grain refinement led to the reduction in solidification cracking and improved the tensile properties of fusion zone compared to the ones obtained by the commercial AA5556 filler. © 2007 Published by Elsevier B.V. Keywords: Al–Zn–Mg alloy; RDE-40; Scandium; GTAW; Solidification cracking; Mechanical properties 1. Introduction Use of high strength, self-ageing, weldable Al–Zn–Mg alloy has found a wide ranging application all over the world for fabrication of portable light weight military structures like bridges, shelters and missile containers [1]. A demand in the early 1960s for lighter and therefore ballistically stronger alu- minum armor led to the introduction of a heat treatable, weldable aluminum–4.5% zinc–2.5% magnesium alloy designated 7039. Alcan Co. developed this further into the slightly stronger and more corrosion-resistant alloy AA7017. The intermediate strength heat-treated alloys AA7020, favored in France and Ger- many, and AA7018 developed by Alcan for use in parts of vehicles most vulnerable to blast attack. Besides, newer 7XXX alloy RDE-40 has been introduced by Research and Develop- ment Engineer Pune in India. This alloy makes it possible to obtain strong improvements in strength and toughness than older alloy AA7020/AA7018. These alloys are known to be suscepti- ble for solidification cracking. By using proper composition of the filler, solidification cracking can be minimized [2]. For this reason these alloys are usually welded with Al–Mg fillers which do not respond to ageing treatment. Apart from the application of Al–Mg fillers, solidification cracking can also be minimized ∗ Corresponding author. Tel.: +91 44 22570752; fax: +91 44 22570509. E-mail address: anandhi.ravidev@gmail.com (Selvi Dev). by grain refinement techniques like pulsing, magnetic arc oscil- lation and inoculation [3]. Traditionally, refinement of most Al alloys relies on the inoculation of the alloy melts with Ti/TiB 2 master alloys. However, while this produces reasonable grain sizes at usual casting rates (100–150 m), their performance is very dependent on the melt contact time and ensuring that poi- soning elements, such as Zr and Si are not present. However, debate still surrounds their exact mechanism of refinement, and there are many problems associated with their use. Further, in the recent past, there is a greater interest in the additions of scan- dium in aluminum alloys, to improve the weldability and also to improve the mechanical properties of the welds. Additions of scandium in aluminum alloys were first used for structural purposes in Soviet aircraft and missile [4]. Literature suggests that it causes a far greater reduction in grain size than is possible with conventional refiners. The beneficial effect of Scandium additions on the fusion zone grain structure and weldability of 7108 alloys has been studied by Mousavi et al. [5]. He observed that grain size, as well as cracking susceptibility, decreased with increasing amounts of scandium and hot cracking was com- pletely eliminated at scandium additions above 0.25 wt%. A more pronounced grain refining effect in welds was observed with Tibor and in addition and no hot cracking was observed with Tibor additions as low as 0.02 wt% Ti. Lathabai et al. proved that 0.17% Sc containing Al–Mg–Sc welds exhibits high yield and tensile strength than that of Sc free alloy of similar composition [6]. Norman et al. reported that the 0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.02.080